Conventionally, a multimode filter also comprises means for coupling energy between modes, said means advantageously being adjustable so as to adjust the transfer of energy between said modes.
One such filter and resonator known in the prior art are described, for example, in patent U.S. Pat. No. 4,489,293 to S. FIEDZIUSZKO, specifically incorporated in the present application for its description of the prior art. In that patent, a filter is constituted by a plurality of composite two-mode resonators disposed in series and coupled together by coupling means, e.g. irises or slots.
The composite resonator of that known device is shown in FIG. 1. It comprises cylindrical pellets 27 of dielectric disposed in a hollow cylindrical cavity 3, 5 with the axes of symmetry of the cavity and of the pellets coinciding.
The cavity is itself of dimensions that are sufficiently small for the intended operating frequency of the composite resonator to be smaller than the cutoff frequency of the cavity in the absence of any dielectric element.
The two-mode filter 1 of the prior art includes two orthogonal modes, and also frequency tuning means for each of these modes, in the present example constituted by tuning screws 29, 31 which project from the walls of the cavity into the inside thereof; these screws are spaced apart on the wall by 90.degree. about the axis of symmetry of the cavity.
In that known device, provision is also made for a coupling screw 33 enabling the transfer of RF energy between the two orthogonal modes to be adjusted, said screw 33 being disposed at 45.degree. to the other two, tuning screws 29, 31.
In spite of the technical and industrial success of the filter disclosed in that prior document, a few practical problems nevertheless remain in its realization and its operation
Firstly, it is quite difficult to position the dielectric cylinder internally since it needs to be held by separate holding elements. The assembly must present good reproducibility and good dimensional accuracy, but without that influencing the RF fields present in the resonator in operation.
Secondly, thermal conductivity from the dielectric resonator to the walls is generally poor, since materials having suitable RF characteristics for making the distinct holding elements are not, in general, good conductors of heat.
Thirdly, the prior art filter remains relatively heavy and bulky, in particular for on-board applications such as communications systems on board satellites, aircraft, or mobile platforms-on land or at sea.